9.4 Induction Motor Common Mode Circuit
The motor has some parasitic capacitances with small values, in the order of picofarads. The CM voltage on the stator windings creates a shaft voltage by capacitive coupling through the motor air gap. Therefore electrostatic discharges are generated through the bearing lubricating film. Such parasitic currents cause a reduction in the lifetime of the bearings. The motor's main parasitic capacitances are presented in Figure 9.8.
In Figure 9.8, the following motor parasitic capacitances are labeled as:
All the parasitic capacitances, except Cph, are significant for the CM. So in the next explanation, the phase-to-phase capacitance Cph is omitted.
The Cwf capacitance is dependent on the motor's mechanical size, according to empirical relation :
where Cwf is in [nF] and motor mechanical size H is in [mm].
The bearing capacitance is dependent on the bearing dimensions and mechanical properties. ...